There is a security interest in detecting threatening materials, such as radioactive materials, explosives, etc., at airports, seaports, border crossings, public places, and other locations. Conventionally, radiation detectors for identifying materials such as radioactive materials, explosives, etc., are effective only in close proximity to the material because radiation intensity drops at a rate proportional to 1/r2, where r is the distance between the material and the radiation detector. Thus, conventional detection methods are relatively ineffective in standoff detection scenarios where detectors are located a substantial distance from containers, cargo, land vehicles and watercraft to be scanned for threatening materials.
Radiation detection methods are primarily reliant on ionization and scintillation processes, which often require high-voltage biasing and/or electro-optical assemblies that may be electrically and mechanically complex. Such radiation detectors may also be too bulky, be too fragile, have a relatively low-power output, and have a relatively low signal to noise ratio (SNR), which often make conventional radiation detectors unsuited for field deployment, where there exists a relatively high level of thermal background and atmospheric attenuation. In addition, such radiation detectors may also require cryogenic cooling.
Inorganic scintillators, for example, exhibit a nonlinear response and poor energy resolution, which limits their use for high-resolution spectroscopic applications. Some conventional radiation detectors require the growth of crystals that are expensive and fragile. Furthermore, many conventional neutron detectors do not have capability to provide imaging or directional information.
Radiation detection in the terahertz (THz) region has faced several issues in terms of conventional THz sources or detectors. The conventional THz sources may include ultra-fast laser switches, pumped gas lasers, optical difference generation techniques, frequency doubling diodes and quantum cascade lasers. Such components may require cumbersome equipment and large power sources. As a result, conventional methods that use time domain THz spectroscopy require a laser scanning system with fragile optical components, do not provide real-time analysis, are expensive, and are not readily adapted to field use. Other radiation detection systems, such as ion mobility spectrometers may require a small sample of the material (e.g., explosive material) to be physically brought to the ion mobility spectrometer for analysis. As a result, conventional methods of radiation detection may be less efficient, relatively expensive, and time consuming.
Turning to another technology, frequency selective surfaces (FSS) are used in a wide variety of applications including radomes, dichroic surfaces, circuit analog absorbers, and meanderline polarizers. An FSS may be any surface construction designed as a “filter” for plane waves with angular/frequency dependence and a bandpass/bandstop behavior. For example, an FSS may comprise a two-dimensional periodic array of electromagnetic antenna elements. Such antenna elements may be in the form of, for example, conductive dipoles, loops, patches, slots or other antenna elements. FSS structures generally include a metallic grid of antenna elements formed on a dielectric substrate. Each of the antenna elements within the grid defines a receiving unit cell. An electromagnetic wave incident on the FSS structure will pass through, be reflected by, or be absorbed by the FSS structure. This behavior of the FSS structure generally depends on the electromagnetic characteristics of the antenna elements, which can act as resonance elements. As a result, the FSS structure can be configured to function as low-pass, high-pass, or dichroic filters. Thus, the antenna elements may be designed with different geometries and different materials to generate different spectral responses.